22 2 JOURNAL OF SCIENCE. 



have passed out of one drainage system into another. The first is 

 characteristic of land-ice, the second of shore-ice, and the third of ice- 

 bergs. Counterfeits : Small erratics may be borne on floating trees 

 or seaweed, consequently their value as evidence of ice depends much 

 on their size. Ships' ballast may also sometimes counterfeit erratics. 

 Ships have been unloaded and careened in many a bay, and have 

 again left with part of their ballast on shore. 



Cirques. The origin of cirques by ice-action is doubted by many; 

 but true cirques appear to be confined to glaciated regions. Very 

 close imitations may, however, be brought about by stream erosion, 

 and occasionally they might be imitated by lateral craters. 



Rock-basins are generally found on a surface which has been 

 formerly glaciated. The terminal moraine of a glacier marks a point 

 in the valley where no erosion is taking place. Below this point the 

 river gradually deepens its channel, while above it the glacier slowly 

 decreases the slope of the valley ; and, however slow a process glacier 

 erosion may be, a rock-basin will, in time, be hollowed out behind the 

 moraine. Rock-basins can also be formed in an arid country by the 

 atmospheric decomposition of level surfaces of rock, the products of 

 decomposition being blown away by the wind. If the climate changes 

 these rock-basins might become lakes ; but, evidently lakes with this 

 origin must be of rare occurrence. Other rock-basins are due to 

 unequal movements of the land. 



Evidence of former Ice-action. 



Evidence of the former presence of glaciers consists of terminal 

 and lateral moraines — especially the former. Roches moutonnees and 

 striations occur in the valleys only. A former ice-sheet is marked by 

 a ground moraine of boulder clay and till with scratched stones, as 

 well as by groovings and striations on plains and the tops of hills. 

 Evidence of ice-bergs consists in large erratics, widely scattered and 

 brought from long distances towards the equator. Former ice-floe is 

 known by partially stratified till with marine shells. 



Evidence of the former presence of glaciers in a country where 

 they no longer exist is not sufficient evidence to prove a former glacial 

 epoch ; for the glaciers may have been due to greater elevation and 

 precipitation of snow. There must also be evidence of a former ice- 

 sheet or floe-ice, widely spread over a large extent of country- A 

 reduction of temperature sufficient to cause a glacial epoch, would 

 necessarily be accompanied by a change in the fauna and flora, and 

 we must expect to find evidence of this also. On the other hand if 

 there is evidence that no extinction of a former flora or fauna and the 

 introduction of newer ones has taken place, then we have a very good 

 reason for concluding that no glacial epoch has occurred. In tertiary, 

 and more particularly post-tertiary times, evidence of a glacial epoch 

 should be abundant and general. In older periods it is more difficult 

 to get evidence, but under any circumstances it is unsafe to rely on a 

 few isolated cases of supposed evidence, which may be deceptive. 



